I am working off memory, so I am probably forgetting some. The ones I remember aren't right in the middle of the action. In Diagon Alley, there are benches at the end near the bathroom, and there is a seating area at the Hopping Pot quick service at the other end near the stage. Many people sit on the edge of the stage when it isn't in use, too.
At Hogsmeade, there are several outside the castle. I have spent many an hour sitting on those while DD rode Flight of the Hippogriff over and over.